Child Domain vs Trust Relationship

So here is the scenario-

We are in the process of centralizing IT to a data center in a single location.  I currently have 12 different operating companies that need a shared security and exchange functionality.  As it stands they are all separate individual domains of varying levels.  There is a company wide accounting system that needs to be integrated with AD currently running in a completely separate domain as well that I would like to see people using their own AD log on info to use.

Here is my question-

Knowing that all the Active directory domains need to be touched regardless to get them all up to a uniform functional level and that there is significant work to be done no matter what, which configuration would be best?  I know there are several points to each one, but I want to make sure I am covering my bases now before choosing a path.  Do I go for a single forest\parent domain?  Or separate domains using trusts between the corporate domain and the operating companies like a spoke and hub config? What are the pros and cons of each?

Thanks-